Interview Q&A / System Design

1. What does '99.9% availability' mean, and how much downtime is that?

   99.9% availability, called three nines, allows about 8.76 hours of downtime a year, roughly 43 minutes a month. Each extra nine cuts the allowed downtime by ten times and costs much more to achieve.

   BeginnerQuick answer1 min read
2. What is the difference between availability and reliability?

   Availability is whether the system is up and responding when asked. Reliability is whether it keeps giving correct results over time without failing. A system can be available yet wrong, so it is available but not reliable.

   BeginnerQuick answer1 min read
3. What is the difference between latency and throughput?

   Latency is how long one request takes; throughput is how many requests the system finishes per unit of time. You can improve one without the other, and pushing throughput too hard often raises latency.

   BeginnerQuick answer1 min read
4. Why do engineers track p95 and p99 latency instead of the average?

   Averages hide slow outliers. p99 means 99% of requests were faster than this value, so it shows what your unluckiest users feel, which is what drives complaints and timeouts.

   BeginnerQuick answer1 min read